The differentiation of cells of the monocytic lineage into mature osteoclasts (OC) is specifically induced by the tumor necrosis factor-related factor, RANKL (receptor activator of NF-B ligand; also known as OPGL, ODF, or TRANCE). Because inhibition of osteoclastogenesis is one of the main mechanisms by which estrogen (E2) prevents bone loss, it is likely that E2 may regulate either the production of, or the target cell responsiveness to RANKL. We found that E2 decreases the differentiation into OC of both murine bone marrow monocytes and RAW 264.7 cells, a monocytic line, by downregulating the activation of Jun N-terminal kinase 1 (JNK1). Diminished JNK1 activity results in decreased nuclear levels of the key osteoclastogenic transcription factors, c-Fos and c-Jun, and lower binding of these transcriptional inducers to DNA. Thus, one novel mechanism by which E2 down-regulates osteoclastogenesis is by decreasing the responsiveness of OC precursors to RANKL.
The differentiation of osteoclast (OC)1 precursors of the monocytic lineage into mature OCs is induced specifically by simultaneous stimulation of bone marrow monocytes by two cytokines produced by bone marrow stromal cells, M-CSF (1-3) and a TNF-related factor known as receptor activator of NF-B ligand (RANKL) (also known as OPGL, TRANCE, or ODF) (4 -6). The formation of mature OCs is completely dependent on the presence of both RANKL and M-CSF, as demonstrated by the lack of OC development in mice lacking the expression of either factor (5,7,8).It is now recognized that inhibition of osteoclastogenesis is the main mechanism by which estrogen (E2) prevents bone loss (9, 10). Moreover, considerable evidence support the hypothesis that E2 down-regulates OC formation by blunting the production of IL-1, IL-6, and TNF (9 -13), cytokines that enhance the stromal cell production of RANKL and M-CSF (14 -17). Additional mechanisms that serve to explain the antiosteoclastogenic effects of E2 include the ability of sex steroids to stimulate the production of osteoprotegerin (OPG), a decoy RANKL receptor (18), and a direct repressive effect on the production of membrane-bound M-CSF (19,20).Despite the relevance of RANKL as an osteoclastogenic factor, and of the potent antiosteoclastogenic effect of E2, sex steroids have not been reported to regulate the production of RANKL. The lack of such regulation is consistent with the absence of both estrogen-responsive elements and binding sequences for E2-regulated transcription factors on the RANKL promoter (21).Binding of RANKL to its signaling receptor RANK results in the activation of Jun N-terminal kinase (JNK), a mitogenactivated protein kinase that increases the trans activation activity and the production of AP-1 transcription factors. We have recently reported that E2 down-regulates TNF-induced JNK activation in monocytic cells (22), and others have shown that OC formation is impaired in mice lacking the AP-1 factors c-Fos and c-Jun (23,24). These data prompted us to investigate the hypothesis that an additional mechani...